This invention relates to motors and actuators. More specifically, this invention relates to the use of smart materials in actuators and motors.
Electric motors are used in an extremely wide variety of applications, and a competitive technology offering significant improvements could have tremendous market potential. Several shortcomings of electric motors include electric field generation, low torque density (torque output per motor volume), heat generation, and a heavy iron core. In certain high performance applications, a higher power density (power output per motor volume) than what is possible with electromagnetic (EM) motors is required. Recently, smart material piezoelectric motors have offered an alternative solution. However, the piezoelectric motor designs, both ultrasonic and quasi-static, have lacked the high torque and high power necessary for many applications. Moreover, the piezoelectric motor designs have generally been high-cost, and have not had good durability. The clamping mechanism of such motors typically has been the source of these limitations.
The phrases xe2x80x9csmart materialxe2x80x9d and xe2x80x9cactive materialxe2x80x9d refer to a broad category of materials able to convert energy (usually electrical) to mechanical energy, and vice-versa. In the context of an actuator, a smart (or active) material is one that can perform mechanical work under action of an applied voltage, charge, magnetic field, or temperature change. The most commonly used smart material is PZT (Lead Zirconate Titanate), which uses the inverse piezoelectric effect to generate strains on the order of 0.1%. Other smart materials include: magnetostrictives, which generate similar strains under action of a magnetic field; shape memory alloys, which generate large strains as a temperature-induced phase change; and electroactive polymers, a more recent category of polymers with piezoelectric characteristics.
In the category of smart materials, piezoelectric materials have garnered the largest share of attention, especially in terms of industrial applications. Piezoelectric ceramics have several redeeming features, namely reliability, high energy density, high bandwidth, high stiffness, low price and accessibility, which make them a natural choice for solid state sensors and actuators.
As a result of xe2x80x9csmart materialxe2x80x9d developments over the past 50 years, a new breed of motor has developed. Commonly known as xe2x80x9csmart material motorsxe2x80x9d or xe2x80x9csolid state motorsxe2x80x9d, designs of these devices have gradually improved and are poised to compete with and surpass the performance of traditional motors (electromagnetic and hydraulic). The field of smart material motors can be subdivided into four categories: inchworm-type linear motors, ultrasonic linear motors, quasi-static rotary motors, and ultrasonic rotary motors.
The first smart material motors were of the inchworm-type. A common design feature of these devices is a quasi-static clamping and advancing of a moving element to generate motion resembling the way an inchworm walks. Since the motion of inchworm-type motors is quasi-static, it is most often used in small, stable, high precision applications with relatively high force and low speed requirements.
A second category of smart material motor is the ultrasonic linear motor. These motors are driven with a low voltage drive signal, usually on the order of 20-100 kHz, and are typified by high speed, low force operation. Motion is generated by exciting a structural resonance of a stator, which in turn generates an elliptic path of motion at the contact between the stator and slider. The elliptic oscillation can be generated several ways: a travelling wave excited on an elastic bar, synthesizing two degenerate standing waves, or synthesizing a standing wave and a nonresonant oscillation. When the stator and slider are pressed together, the elliptic motion on the stator pushes the slider in one direction.
A number of developments have also been made in the field of smart material rotary motors. While ultrasonic rotary motors have received most of the attention, a number of recent designs have applied the technology of the quasi-static inchworm concept to rotary motors. These motors are exceptional in terms of torque output, although usually quite slow.
Ultrasonic rotary motors, driven at resonance at frequencies ranging from 20 to 100 kHz, are more common in the literature than the three types presented so far. As small, lightweight, quiet alternatives to electromagnetic motors, ultrasonic motors are used in industrial applications such as camera lenses, printers, and floppy disk drives. Ultrasonic rotary motors, like ultrasonic linear motors, are divided into two classes by their mode of operation: standing-wave type, and propagating-wave type. Standing-wave motors are driven with a single frequency input and combine two excited resonant mode shapes, generating an elliptical path to drive a rotor. These motors have the potential of being low cost and highly efficient, but are limited to uni-directional motion. In contrast, propagating-wave motors require two vibration sources to generate two standing waves. The two standing waves are superimposed to form a propagating wave which generates elliptical motion and drives the rotor. Propagating-wave rotary motors have a lower potential efficiency, but do offer reversibility.
In the past, ultrasonic rotary motors have been used primarily in small, low torque positioning applications. Recently, some designs have offered high torque and power performance, but still have high energy losses due to friction.
Consulting a catalog of commercial motors under 100 hp (75 kW) leads to a rough power density of 100 W/kg for typical electric motors. Electric motors and piezoelectric material-based motors have an advantage over hydraulic systems in that power may be transferred over long distances with relatively light wires. As a rule, piezoelectric material-based motors have advantages over typical electric motors in that they offer better potential to conform with geometric requirements associated with tightly-integrated adaptive structures, and in the potential for reduced electromagnetic field generation.
Numerous problems exist regarding motors and actuators. These problems include the need for high specific power, reliability, life, and efficiency. Although these same problems exist in numerous applications, one specific area where these problems surface is in military applications. For example, although the military may be able to track moving targets it lacks the ability to pursue them. Agile high-speed weapons, for both air and marine systems, would address such a need and represent a new military capability. Compact, conformable high-power actuators are needed to enable such systems. High power density enables the high-bandwidth fin or thrust vectoring control needed for agility, while conformability enables fit into confined spaces. Novel actuators could increase control effectiveness for existing naval undersea weapons as well: current actuators use 1970s or 80s hydraulic or electric technology.
Thus, despite the recognition of the potential of high force smart material actuators, and in particular high force smart material actuators, problems remain.
Therefore it is a primary object of the present invention to improve upon the state of the art.
It is a further object of the present invention to convert the low strain of smart materials to large and useful displacements.
It is a further object of the present invention to overcome the limitation of dynamic clamping force.
It is a further object of the present invention to convert oscillating motion into output mechanical power.
It is a further object of the present invention to provide for a system that achieves high efficiencies.
A still further object of the present invention is to provide an actuator capable of high torque.
A further object of the present invention is to provide an actuator capable of high power.
Yet another object of the present invention is to provide an actuator which is not prohibitively expensive.
Another object of the present invention is to provide an actuator that is durable.
A further object of the present invention is to provide an actuator having a high power density.
A still further object of the present invention is to provide an actuator capable of high speeds.
A still further object of the present invention is to provide a bi-directional actuator.
Yet another object of the present invention is to provide a motor capable of conforming with geometric requirements.
Other objects, features, or advantages of the invention will become apparent from the description of the invention and that which follows.
A novel actuator that uses a resonant system with a smart material drive element is disclosed. The actuator includes one or more mechanical diodes in combination with one or more smart materials. The mechanical diode is used to rectify the motion of high frequency resonant oscillation of the smart material drive element, to convert oscillating motion into output mechanical power.
The invention is a combination of smart materials and mechanical diodes in a resonant system to convert oscillating motion and output mechanical power. The invention includes different types of mechanical diodes, different numbers of mechanical diodes, different numbers of drive elements, and different configurations of one or more mechanical diodes in combination with one or more drive elements. The invention includes both linear drive motors as well as rotary drive motors. Mechanical diodes used may be either linear mechanical diodes or rotary mechanical diodes. Furthermore, the mechanical diodes may be either uni-directional or bi-directional.